Such breath alcohol measuring devices are known, for example, from DE 43 273 312 C2 and U.S. Pat. No. 4,770,026. The pressure sensor is needed to generate a signal representative of the tidal volume flow. This is needed, on the one hand, to make it possible to monitor the interruption-free expiration by the test subject. Furthermore, the tidal volume flow signal is integrated in order for a preset minimum tidal volume to be able to be determined, which is needed for a reliable measuring result, because a sufficient percentage of breathing air is needed from the depth of the lungs to make it possible to infer the blood alcohol concentration from the breath alcohol concentration.
The electrochemical sensor is exposed to a preset tidal volume sample. Ethanol is selectively reacted at the electrodes of the electrochemical sensor, while an electric current is generated, which subsides slowly, following a rapid rise, which corresponds to the subsiding reaction. The sensor current is integrated over a preset period of time, and the corresponding overall load is estimated from this integrated load value, from which the breath alcohol concentration can be deduced.
In the prior-art breath alcohol measuring devices, the signals of the pressure sensor and of the electrochemical sensor are first amplified in a pre-amplifier or operational amplifier and subsequently sent to an analog-digital converter with a range of 10 bits or 12 bits. The analog processing of the sensor signals prior to their digitization entails various drawbacks.
The electrochemical sensor is connected with a current-voltage converter, which is formed by an operational amplifier fed back to one of its inputs via a resistor. When the supply voltage rises, i.e., when the device is switched on, the operational amplifier passes through non-equilibrium states. As a result, a current is briefly sent to the electrochemical sensor, as a result of which a potential is built up in the sensor. Since it may take about 20 sec until this potential is gradually eliminated again, this leads to a corresponding waiting time until the device becomes ready for use after it had been switched on.
When the device is not switched on, the sensor is not short-circuited, either. Due to thermal effects or gases in the ambient air, a potential may build up, which must first be eliminated after switching on, which in turn leads to a longer waiting time.
Another drawback of the prior-art measuring devices is that a plurality of analog-digital channels with different amplifications are frequently needed to cover the necessary dynamic range, because the evaluating means require a resolution of 7 bits for the integration of the signals of the electrochemical sensor. The ratio of the greatest signal (5 promille at 50° C.) to the lowest signal (0.1 promille at −5° C.) is approx. 1,000/1 (10 bits). The signal of the electrochemical sensor must therefore be distributed in the 10-bit analog-digital converters used typically among a plurality of analog-digital channels with different amplifications in order to cover the dynamic range. This is associated with a number of drawbacks, namely, the need for an increased number of components, increased energy consumption and more complicated signal processing.
Another drawback of the prior-art processing of the analog signals in pre-amplifiers is that electromagnetic radiation may affect the measurements. Electromagnetic radiation causes disturbances in the input circuits of the amplifiers, and these disturbances will be amplified as well and will then cause erroneous measuring results.
The following shall be pointed out in reference to the pressure sensor signal processing. The pressure sensor measures the dynamic pressure generated in the mouthpiece. This dynamic pressure is subsequently converted into a tidal volume flow. Since the lowest detectable tidal volume flow should be approx. 3 L/minute, but the highest approx. 50 L/minute, a very broad dynamic range is obtained here. Furthermore, pressure sensors are designed in the form of a bridge circuit, which already delivers a static offset voltage even without pressure. This static offset voltage limits the possible amplification of the pressure signal to a factor of about 100 in real systems, which will in turn lead to an insufficient digital resolution at low tidal volume flows. Since calibrating systems deliver mostly only a weak current, special mouthpieces will be needed for the calibration. As in the case of the electrochemical sensor, the amplifiers used for the pressure sensor are also susceptible to electromagnetic disturbances.